Integrated circuits including a semiconductor die, such as charge-coupled-devices (CCD) and complementary metal oxide semiconductor (CMOS) sensors, have commonly been used in photo-imaging applications. Such sensors typically contain thousands or millions of pixel cells in a pixel array on a single chip. Pixel cells convert light into an electrical signal that can then be stored and recalled by an electrical device such as, for example, a processor.
Semiconductor dies typically are packaged and inserted into imaging devices such as, for example, digital cameras. FIG. 1 illustrates a conventional image sensor module 2, shown in an exploded perspective view. Module 2 includes a semiconductor die 4 disposed on a cushion 6 and having series of bump pad connectors 8 located adjacent the four edges of the top surface of the semiconductor die 4. Semiconductor die 4 includes a pixel array 5. Conductive film 10 connects flex tape 12 to die 4 and connectors 8. Flex tape 12 contains electrical traces that link to a connector 14, and has an opening 16 through which light travels to the pixel array. A stiffener 18 provides additional support to the module 2. An infrared (IR) filter 20 may be provided which is aligned in the optical path between the pixel array and a lens assembly 22 and is adhered onto the flex tape 12. The lens assembly 22 includes a housing 24 supporting a lens barrel 26 containing a lens system for focusing light onto the pixel array. The lens system is aligned within the lens barrel 26 and is secured at a focusing distance from the surface of the pixel array located on the semiconductor die 4 in order to provide a sharp image.
To increase the pixel response to illumination, an array of micro-lenses 32 may be placed above each photosensitive pixel cell 28 of pixel array 5, as shown in the imager 50 of FIGS. 2A and 2B. The micro-lenses 32 are used to focus electromagnetic radiation onto the photo-conversion device, e.g., a photodiode of the pixel cells 28. The centers of the micro-lenses 32 in the array may be aligned with the center of the corresponding pixels 28 or shifted from the center of the corresponding pixels 28 to shelter direct light on the photosensor and to accommodate for predominant incident light angle. This angle is usually characterized by the Chief Ray Angle (CRA) of an optical system and depends on the location of the pixel relative to the optical center of the module.
As semiconductor-based image sensors are being used in even smaller and/or mobile applications, such as digital cameras and cellular phones, the required sensor pixel size is reduced to sizes smaller than 3 μm. This size reduction puts tighter requirements on alignment accuracy and dimensional stability of the imaging lens within lens barrel 26. Typically, the focal plane of the lens should not shift more than ±10 μm from focus position. Plastic lenses and mounts, typically used in current lens assemblies, show a strong variation of indices of refraction and mechanical dimensions depending upon the temperature, therefore the desired small focus shift is difficult to maintain while using plastic lenses. Glass lenses provide a much higher level of stability, but the cost is also much higher. The high cost of known methods of producing glass lenses can be prohibitive for implementing glass lenses in mobile image sensors. Accordingly, it is desirable to find an alternative low-cost solution for manufacturing a stable glass imaging lens system.